This invention pertains generally to the measurement of capacitance and more particularly to a method and apparatus for measuring the capacitance of an element such as a telephone cable.
In the past, there have been attempts to determine the location of an open circuit fault in a communication cable such as a telephone cable by measuring the capacitance of the cable from a known point to the open circuit and dividing the measured capacitance by the capacitance per unit length to determine the distance to the fault.
Heretofore, this approach has been subject to errors and limitations due to the presence of other circuit elements and foreign currents in the cable. Shunt conductance can arise, for example, by water entering a damaged cable and forming conductive paths between the conductors, and high series resistances can arise from conductors which are partly opened. Both the shunt conductance and the series resistance can adversely affect the measurement, and with shunt conductance, the measurements are subject to further error due to currents bridging from the undamaged conductors to the damaged conductors. Another source of foreign currents and error is alternating current induced in the cable from nearby power distribution lines.